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Journal of Electrical, Electronics and Informatics, Vol. 2 No. 2, August 2018
25
Abstract Power transformer is very important in electric power system due to its function to raise or lower the voltage according
to its designation. On the power side, the power transformer serves to raise voltage to be transmitted to the transmission line. On
the transmission side, the power transformer serves to distribute the voltage between the main substations or down to the
distribution voltage. On the distribution side, the stresses are channeled to large customers or lowered to serve small and medium
customers. As the power transformer is so importance, it is necessary to protect against disturbance, as well as routine and
periodic maintenance, so that the power transformer can operate in accordance with the planned time. Some factors that affect the
duration of the power transformer is the ambient temperature, transformer oil temperature, and the pattern of load. Load that
exceeds the maximum efficiency of the transformer which is 80% of its capacity will cause an increase in transformer oil
temperature. Transformer oil, other than as a cooling medium also serves as an insulator. Increasing the temperature of
transformer oil will affect its ability as an isolator that is to isolate the parts that are held in the transformer, such as iron core and
the coils. If this is prolonged and not handled properly, it will lead to failure / breakdown of insulation resulting in short circuit
between parts so that the power transformer will be damaged. PLN data indicates that the power transformer is still burdened
exceeding maximum efficiency especially operating in the work area of PLN South Bali Area. The results of this study, on
distribution transformers with different loads, in DS 137, DS 263 and DS 363, show that DS 363 transformer with loading above
80% has the shortest residual life time compared to DS 263 and DS 137 which loading less than 80%.
Index Terms— Current, Voltage, Duration of Time, Distribution Transformer.
I. INTRODUCTION1
Power transformer is used to transform power. In power
plant, this is used to increase power transferred to
transmission line. In transmission line, however, this is used
to transfer power between substations. In the distribution
side, power is distributed to main customers or reduced to
become low power to serve middle-level or low-level
customer. As power transformer is so importance, it is
highly required to protect and maintain it, therefore, their
lifetime can be extended.
There are factors to influence lifetime of power
transformer: ambient temperature, transformer’s oil
temperature, and loading pattern. Loading process in
distribution transformer influences voltage and current in
transformer’s coil. In ideal condition, transformer is loaded
up to 80% of its capacity due to its efficiency and losses.
Power transformer management is related to the routine
maintenance, for preventive, corrective, or detective.
Shorten lifetime or damaged transformer caused by isolation
fail in parts of transformer such as iron core and coils. This
can happen due to the function of isolation to break parts
with voltage to avoid flash over or spark over. Isolation fail
can be caused by some factors, such as time, reduced-
1
Electrical Engeneering Departement, Faculty of Engeneering
Udayana University (UNUD)
Badung, Bali, Indonesia
ngr_amrita@unud.ac.id,
2Electrical Engeneering Departement, Faculty of Engeneering
Udayana University (UNUD)
Badung, Bali, Indonesia
w.ariastina@unud.ac.id,
3 Electrical Engeneering Departement, Faculty of Engeneering
Udayana University (UNUD)
Badung, Bali, Indonesia
ibgmanuaba@unud.ac.id
Study of Transformer Lifetime Due to Loading
Process on 20 KV Distribution Line
A.A.N. Amrita
1
, W.G. Ariastina
2
, and I.B.G. Manuaba
3
Journal of Electrical, Electronics and Informatics, p-ISSN: 2549 - 8304 e-ISSN : 2622 - 0393
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dielectric strength, extra voltage and high current. Extra
voltage in isolator, is a stress that must be counter by
isolator for not being fail when isolating current. Inside
isolation, electrons hardly link to their molecule, where this
link forces stress caused by voltage. If this link is broken,
isolation is disappeared, and it is fulfilled with power, there
are moving electrons from one molecule to the others,
where therefore, leaked current is created. On the other
hand, High voltage that constantly flows inside coils cause
temperature escalation.
PLN Area Bali Selatan says that in 2016, there are
distribution transformers with load more than 80%, exceeds
their capacity. Based on these facts, this research aims to
analyze effect of loading to life time of power transformer
by comparing power transformer with load of 50% - 60%,
70% - 80%, and 80% - 90% of its capacity. Results of this
research can be used as a base of power transformer
replacement or mutation, therefore power transformer
damage that influences service continuity can be minimized.
There are previous studies related to power transformer
lifetime and its load factor:
1. Janny Olny Wuwung, 2010. Here, effect of loading
process to temperature increment in oil-submerged
transformer is studied. Results of this study show
that increment of loading factor raises aging factor
that decrease the lifetime of a transformer.
2. Syafriyudin, 2011. In this study, transformer time of
use prediction is calculated from daily voltage and
current. Results of this study is that time of use of a
transformer is reduced 8 months out of its normal
time.
3. Sofyan and Afryastuti Herawati, 2015, studied effect
of, loading process to efficiency and lifetime of
transformer based on IEC 60076-7. Results of their
research show that the highest efficiency of
96.66% occurs in peak load.
4. Winarso, 2014. Time of use of a transformer is
predicted from daily voltage and current
calculation. Results of this paper shows that for
transformer with 5 years lifetime, it can be used for
only 4 years and 7 months.
II. BASIC CONCEPT
Transformer is a robust magnetoelectric tool used to
change voltage. In general, Transformer composed by core
formed by iron plate and two coils, primary and secondary
coil. Ratio of voltage change depends ratio of those two
coils (Abdul Kadir). Turn ratio of transformer is a deciding
factor of the transformer type. If the turn ratio is lower than
one, it is a step-up transformer. Otherwise, turn ratio that is
higher than one means that the transformer is a step down
one (Sumanto).
Distribution transformer used to distribute power to
customers. A proper election of distribution transformer
capacity is highly decided by the supplied load; therefore, it
can support the continuity of service, reliability, and
transformer lifetime. Distribution transformer reach
maximum efficiency when it is loaded until 80 percent of its
capacity. When the load is too high, it is required to change
the transformer, to insert new transformer, or to shift
transformer (low-loaded transformer is shifted to serve
high-loaded transformer or vice versa). Equation 1 is used
to calculate distribution transformer rating where pf is
power factor (cos phi = 0.85).
Distribution transformer rating (KVA) = Load (KW)/pf (1)
Prediction of Transformer lifetime can be analyzed
through error value of transformer load. Error value of
transformer is counted by using Equation 2.
% 100
*x
P
PP
E
−
= (2)
where :
E = Error value ( % )
P = Threshold value of transformer usage (80%)
P* = I ( % ) + V ( % ) + P ( % )
Meanwhile, each percentage value of I, V, and P can be
calsulated by these methods:
%)( 100
(%) x
Inom
IbebanInom
I
−
= (3)
%)( 100 (%) x
Vinput
VoutputVinput
V
−
=(4)
%)( 100 (%) x
Pnom
PbebanPnomi
P
−
=(5)
V
S
Inom 3
= (6)
Where :
S = Pnom = Power of power transformer ( KVA )
Inom = Phase current (A)
V = Vinput = Phase voltage ( V )
Voutput = VL = Load voltage from experiment ( V )
Ibeban = IL = Current from experiment ( A )
Pbeban = Load power ( KW )
Load power is calculated by using equation 7.
φ
cos3 LLbeban IVP = (7)
Journal of Electrical, Electronics and Informatics, Vol. 2 No. 2, August 2018
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If load current IL is current that transfers power to load in
a balance mode, therefore, in the curent power
transmission with unbalance mode, value of load current
can be stated with a, b, and c coefficients as it are shown
in equation 8 where IR, IS, dan IT are load current in each
phase, R, S, and T.
[
]
[
]
[ ] [ ]
[ ] [ ]
IaI
IaI
IaI
T
S
R
=
=
=
(8)
If power factors in these three phases is equal, even each
current is different, transmitted power can be stated by
equation 9.
φ
cos3 )(
LLbeban
IVcbaP ++= (9)
Due to the imbalanced load in each phase of secondary
transformer, it is current to traverse the neutral of
transformer, where this current causes losses. Losses
value in the neutral of transformer can be calculated using
equation 10. In this equation, PN is losses in the neutral
conductor of transformer (Watt), IN is current arises in
neutral conductor (Ampere), and RN is resistance in
neutral conductor (Ohm)
NNN
RIP
2
= (10)
Afterwards, percentage of transformer lifetime is
calculated through equation 11
Transformer lifetime (%) = 100(%) – E(%) (11)
If it is predicted that transformer can be used until 10
years, therefore its lifetime is calculated as folows.
Lifetime (day) is lifetime(%) multiply by 3600 days. Or it
can be calculated by lifetime (year) which is lifetime(day)
divide by 3600 days.
III. METHODOLOGY
This research was conducted in three distribution
transformers: DS 137 located in Tirta Nadi street, DS 263 in
Danau Tamblingan street, and DS 363 which is located in
Pulau Serangan street. These transformers are selected due
to the early data from PT. PLN Bali South Regian in 2016.,
which is corresponding to the loading criteria of this
research, 50% - 60%, 70% - 80%, and 80% - 90%. Data of
each transformer is:
•DS 137 brand of B.D, installed in 2011, capacity
of 250 KVA, 380/220 V, load of 47.91%.
•DS 263 brand of Unindo, installed in 2011,
capacity of 250 KVA, 380/220 V, load of
71.37%.
•DS 363 brand of Trafindo, installed in 2011,
capacity of 250 KVA, 380/220 V, load of
83.24%.
Measurement was conducted from 19.00 until 20.00,
which is time of peak load. This schedule was repeated for
seven days. The tool used is Clamp Power Meter.
IV. RESULTS
Results of our research are shown in Table 1, 2, and 3,
for DS 137, DS 263, and DS 363 respectively.
Table 1
Measurement results of DS 137
IR IS IT IN RS RT ST RN SN TN
1163 178 185 57 391 401 398 227 225 231
2184 193 205 47 391 400 397 227 225 229
3114 124 223 95 398 390 400 223 231 227
498 67 81 37 403 397 409 228 235 231
5193 151 163 58 387 398 395 225 223 229
6193 151 163 58 387 398 395 225 223 229
7123 48 122 80 394 387 401 223 230 226
Total 1068 912 1142 432 2751 2771 2795 1578 1592 1602
Ave rage 152.57 130.2 9 163.14 61.7 14 393 395 .86 399 .29 225 .43 227 .43 228 .86
Day
MEASUREMENT RESULTS
CURRENT (AMPERE) VOLTAGE ( VOLT )
PHASE NEUTRAL PHAS E
Table 2
Measurement results of DS 263
IR IS IT IN RS RT ST RN SN TN
1
282 265 251 46 385 397 392 223 221 227
2
196 246 260 90 383 394 391 223 220 225
3
199 216 269 91 395 387 397 224 226 225
4
324 257 186 143 389 403 398 227 225 230
5
259 258 273 112 388 380 397 220 227 226
6
266 339 266 125 380 397 387 224 218 227
7
267 197 303 164 388 380 396 219 229 224
Total
1793 17 78 180 8 77 1 2708 2 738 275 8 1560 15 66 158 4
Ave rage
256.14 25 4.00 258 .29 110 .14 386 .86 391 .14 394 .00 222.8 6 223.7 1 226 .29
DAY
MEASUREMENT RESULTS
CURRENT (AMPERE) VOLTAGE ( VOLT )
PHASE NEUTRAL PHASE
Table 3
Measurement results of DS 363
IR IS IT IN RS RT S T RN SN TN
1 323 301 284 134 402 397 405 224 233 231
2
300 362 318 103 391 387 397 223 227 224
3
253 339 375 158 399 404 401 232 228 232
4
291 370 306 123 396 390 398 225 226 228
5
300 318 384 135 391 383 396 222 228 223
6
389 296 401 130 390 398 395 228 224 227
7
375 378 308 174 395 392 401 224 230 228
Total 2231 23 64 237 6 95 7 2764 2 751 27 93 1578 1 596 15 93
Ave rage 318.71 337.7 1 339.4 3 136 .71 394.8 6 393 39 9.00 22 5.43 228 2 27.57
DAY CURRENT (AMPERE) VOLTAGE ( VOLT )
PHASE NEUTRAL PHASE
MEASUREMENT RESULTS
V. DISCUSSION
Results of lifetime calculation in each distribution
transformer are shown in Table 4, 5, and 6, for DS 137, DS
263, and DS 363 respectively.
Journal of Electrical, Electronics and Informatics, p-ISSN: 2549 - 8304 e-ISSN : 2622 - 0393
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Table 4
DS 137 with 50% - 60% load
Pnominal, (250 KVA = 250000 VA) 250,000.00 VA
294,117.65 Watt
Power faktor 0.85
V input 220.00 V/phase
I nominal (eq. 6) 379.69 A/phase
I load 148.67 A/phase
Voutput 227.24 V/phase
Pload (eq. 7) 49,677.50 Watt
49.68 KW
Percentage value :
I (%), eq. 3 60.84 %
V (%), eq. 4 (3.29) %
P (%), eq. 5 83.11 %
P*, e1. 6 140.66 %
Error, eq 1 (75.83) %
Lifetime, eq 11 175.83 %
Lifetime in day 6,329.90 day
Lifetime in year 17.58 year
Table 5. DS 263 with 70% - 80% load
Pnominal, (250 KVA = 250000 VA) 250,000.00 VA
294,117.65 Watt
Power faktor 0.85
V input 220.00 V/phase
I nominal (eq. 6) 379.69 A/phase
I load 256.14 A/phase
Voutput 224.29 V/phase
Pload (eq. 7) 84,479.02 Watt
84.48 KW
Percentage value :
I (%), eq. 3 32.54 %
V (%), eq. 4 (1.95) %
P (%), eq. 5 71.28 %
P*, e1. 6 101. 87 %
Error, eq 1 (27. 33) %
Lifetime, eq 11 127.33 %
Lifetime in day 4,584.03 day
Lifetime in year 12.73 year
Table 6
DS 363 with 80% - 90% load
Pnominal, (250 KVA = 250000 VA) 250,000.00 VA
294,117.65 Watt
Power faktor 0.85
V input 220.00 V/phase
I nominal (eq. 6) 379.69 A/phase
I load 331.95 A/phase
Voutput 227.00 V/phase
Pload (eq. 7) 110,806.87 Watt
110.81 KW
Percentage value :
I (%), eq. 3 12.57 %
V (%), eq. 4 (3.18) %
P (%), eq. 5 62.33 %
P*, e1. 6 71.72 %
Error, eq 1 10.36 %
Lifetime, eq 11 89.64 %
Lifetime in day 3,227.21 day
Lifetime in year 8.96 year
These calculation results show that the higher the given
load, affects to the diminished of lifetime of distribution
transformer. Finally, Figure 1 shows the relations between
lifetime and loading factor of power transformer.
Fig. 1. Relation between loading factor and lifetime.
VI. CONCLUSIONS
Loading in power transformer affects lifetime. The higher
the load the shorter the lifetime. If it is assumed that power
transformer can be used for 10 years, transformer DS 137
with brand of B.D then can be used for 11.58 years when it
is loaded only 50% to 60% of its capacity. Meanwhile, DS
263 with brand of Unindo with load of 70% to 80% of its
capacity, can be used for 6.73 years. Lastly, DS 363 with
brand of Trafindo, where it is loaded with 80% to 90% of
its capacity, its lifetime is only 2.96 years.
VII. ACKNOWLEDGMENT
We would like to express our sincere gratitude to:
1. Rector of Udayana University, through Dean of Faculty
of Engineering for the grant with value of 25.000.000
IDR, where therefore this research can be finished.
2. Head and secretary of the Electrical Engineering
Department, students, colleges who have collaborated to
finish this research.
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